Here is an example of c-erb-B2 (HER2) positivity of the neoplastic cells in a breast carcinoma. This oncogene acts via reduplication of the normal proto-oncogene hundreds of times, leading to production of a protein product that drives unregulated cell growth. This is detected here by immunohistochemical staining, with the brown reaction product concentrated in a perimembranous pattern around the neoplastic cells.